A method and an apparatus (referred to as a prior art hereinafter) for extending a band of an audio signal are disclosed in, for example, a pamphlet of international application publication No. WO00/70769. According to the prior art, a higher harmonic wave component is generated based on an inputted audio signal, a level of the inputted audio signal is detected, and a noise signal, which is a random higher harmonic wave component, is generated independently of the inputted audio signal. Then, after a level of a generated noise signal is changed according to a detected level, a generated higher harmonic wave component is added to a level-changed noise signal, and a predetermined bandpass filtering processing is executed on a signal having an addition result. Further, the inputted audio signal is added to a signal on which the bandpass filtering processing has been executed while adjusting the level of the inputted audio signal, and a signal having an addition result is outputted as an outputted signal from the apparatus.